There are currently in existence spectacles of photochromic type, in which the lenses are constructed in materials which vary their own transparency as a function of the intensity of the irradiation received. This permits the user to have a lens which has a greater or lesser screening effect, depending upon the external conditions. Spectacles of this type exhibit the disadvantage that the chromatic variation of the materials used is rather slow and thus a great length of time is necessary for the adaptation of the lens to the external irradiation conditions. This disadvantage has placed considerable limitations on the widespread use of this type of spectacles, which are used only in a few applications, for example in the field of sports, where it is possible to find skiing goggles of photochromic type.